Composites are important engineering materials that combine the properties of multiple components to afford materials with new properties that are not attainable in the individual components. The use of nanoscale components, or nanofillers, in composite structures has been shown to significantly affect mechanical, dynamic, optical, fire resistance, and electrical and thermal conduction properties. To optimize the properties of composite materials, control over their structure (location and orientation of filler materials) is important. Many current techniques for the production of nanocomposite materials result in isotropically random materials (no preferred location and orientation of the filler materials and thus do not take advantage of the alignment of the nanofillers) with low concentrations of the nanofiller, which places constraints on the types of materials that can be formed. Property enhancements could be significantly improved if control over their structure and increases in nanofiller content could be attained.
For example, traditional composites comprise high-aspect-ratio fillers at high concentrations (>50 wt %), which tend to feature anisotropic orientation of the filler phase due to their inability to pack isotropically (rods with aspect ratio 500 begin to order at ˜6 vol %) (see E. A. Dimarzio, A. J. M. Yang, S. C. Glotzer. J. Res. Natl. Inst. Stan. 1995, 100, 2). This anisotropic orientation affords significant property enhancements in the plane of alignment, but less pronounced effects in other orthogonal planes, which can be of advantage in certain applications. Such high additive contents, however, have not been extensively explored in polymer nanocomposites due to high costs of the nanofillers and processing difficulties (e.g., particle aggregation).
The most frequent method for the production of nanocomposites with high (˜50 wt %) loadings of nanoparticles, such as those containing clay and carbon nanotubes, is layer-by-layer assembly (LBL). This technique requires the methodical layering of polymers and nanoparticles via exposure of a substrate to alternating solutions of the composite components, typically performed by a robotic setup. The formation of the layered structure is induced during assembly by strong attractions between the individual components, which are also responsible for reinforcing the final structure, leading to impressively strong materials. While LBL can produce layered nanocomposites with excellent mechanical properties, it has a few drawbacks: limited material selection (water solubility of all components, strong attraction between components), setup cost and complexity, fabrication speed, and a narrow range of interlayer polymer composition.
Non-composite inorganic “paper-like” materials based on nanoscale components such as exfoliated vermiculite or mica platelets have been intensively studied and commercialized as protective coatings, high temperature binders, dielectric barriers, and gas-impermeable membranes. Carbon-based flexible graphite foils composed of stacked platelets of expanded graphite, have long been used in packing and gasketing applications due to their chemical resistivity against most media, superior sealability over a wide temperature range, and impermeability to fluids. The discovery of carbon nanotubes brought about bucky paper, which displays excellent mechanical and electrical properties that make it potentially suitable for fuel cell and structural composite applications, among others.
Graphite oxide (GO) is a layered material consisting of hydrophilic oxygenated graphene sheets (graphene oxide sheets) bearing oxygen functional groups on their basal planes and edges. GO-based thin films had been fabricated via solvent-casting methods as described by Titelman et al., “Characteristics and microstructures of aqueous colloidal dispersions of graphite oxide”, Carbon 43, 641-649 (2005).